Muscle stimulator

ABSTRACT

An implantable medical device for treating the back of a patient. Stimulation energy is delivered to muscles or joint capsules or ligaments or nerve fibers to improve the heath of the back.

CROSS REFERENCE TO RELATED CASES

This case claims the benefit of and incorporates by referenceProvisional Application 60/905,979 filed Mar. 9, 2007.

FIELD OF THE INVENTION

The present invention relates to medical devices and more particularlyto a stimulator for treating muscles and neural pathways in the back.

BACKGROUND OF THE INVENTION

The human back is a complicated structure including bones, muscles,ligaments, tendons, nerves and other structures. The spinal columnconsists of interleaved vertebral bodies and intervertebral discs. Thesejoints are capable of motion in several planes includingflexion-extension, lateral bending, axial rotation, longitudinal axialdistraction-compression, anterior-posterior sagittal translation, andleft-right horizontal translation. The spine provides connection pointsfor a complex collection of muscles that are subject to both voluntaryand involuntary control.

Back pain is common and recurrent back pain in the lower or lumbarregion of the back is well documented. The exact cause of most back painremains unproven. One common notion is that some cases of back pain arecaused by abnormal mechanics of the spinal column. Degenerative changes,injury of the ligaments, acute trauma, or repetitive microtrauma maylead to back pain via inflammation, biochemical and nutritional changes,immunological factors, changes in the structure or material of theendplates or discs, and pathology of neural structures.

The spinal stabilization system was conceptualized by Manohar Panjabi toconsist of three subsystems: 1) the spinal column, to provide intrinsicmechanical stability; 2) spinal muscles surrounding the spinal column toprovide dynamic stability; and 3) the neuromotor control unit toevaluate and determine requirements for stability via a coordinatedmuscle response. In patients with a functional stabilization system, thethree subsystems work together to provide mechanical stability.

The spinal column consists of vertebrae and ligaments (e.g. spinalligaments, disc annulus, and facet capsules). There is an abundance ofin-vitro work in explanted cadaver spines and models evaluating therelative contribution of various spinal column structures to stability,and how compromise of a specific column structure will lead to changesin the range of motion of spinal motion segments.

The spinal column also has a transducer function, to generate signalsdescribing spinal posture, motions, and loads via mechanoreceptorspresent in the ligaments, facet capsules, disc annulus, and otherconnective tissues. These mechanoreceptors provide information to theneuromuscular control unit, which generates muscle response patterns toactivate and coordinate the spinal muscles to provide muscle mechanicalstability. Ligament injury, fatigue, and viscoelastic creep may corruptsignal transduction. If spinal column structure is compromised, due toinjury, degeneration, or viscoelastic creep, then muscular stability isincreased to compensate and maintain stability.

Muscles provide mechanical stability to the spinal column. This isapparent by viewing cross section images of the spine, as the total areaof the cross sections of the muscles surrounding the spinal column ismuch bigger than the spinal column itself. Additionally, the muscleshave much larger lever arms than those of the intervertebral disc andligaments

Under normal circumstances, the mechanoreceptors generate signals to theneuromuscular control unit for interpretation and action. Theneuromuscular control unit produces a muscle response pattern based uponseveral factors, including the need for spinal stability, posturalcontrol, balance, and stress reduction on various spinal components.

It is believed that in some patients with back pain, the spinalstabilization system is dysfunctional. With soft tissue injury,mechanoreceptors may produce corrupted signals about vertebral position,motion, or loads, leading to an inappropriate muscle response. Inaddition, muscles themselves may be injured, fatigued, atrophied, orlose their strength, thus aggravating dysfunction of the spinalstabilization system. Conversely, muscles can disrupt the spinalstabilization system by going into spasm, contracting when they shouldremain silent, or contracting out of sequence with other muscles. Asmuscles participate in the feedback loop via mechanoreceptors in theform of muscle spindles and golgi tendon organs, muscle dysfunctioncould further compromise normal muscle activation patterns via thefeedback loops.

Trunk muscles may be categorized into local and global muscles. Thelocal muscle system includes deep muscles, and portions of some musclesthat have their origin or insertion on the vertebrae. These localmuscles control the stiffness and intervertebral relationship of thespinal segments. They provide an efficient mechanism to fine-tune thecontrol of intervertebral motion. The lumbar multifidus, with itsvertebra-to-vertebra attachments is an example of a muscle of the localsystem. Another example is the transverse abdominis, with its directattachments to the lumbar vertebrae through the thoracolumbar fascia.

The multifidus is the largest and most medial of the lumbar backmuscles. It consists of a repeating series of fascicles which stem fromthe laminae and spinous processes of the vertebrae, and exhibit aconstant pattern of attachments caudally. These fascicles are arrangedin five overlapping groups such that each of the five lumbar vertebraegives rise to one of these groups. At each segmental level, a fasciclearises from the base and caudolateral edge of the spinous process, andseveral fascicles arise, by way of a common tendon, from the caudal tipof the spinous process. Although confluent with one another at theirorigin, the fascicles in each group diverge caudally to assume separateattachments to the mamillary processes, the iliac crest, and the sacrum.Some of the deep fibers of the fascicles which attach to the mamillaryprocesses attach to the capsules of the facet joints next to themamillary processes. All the fasicles arriving from the spinous processof a given vertebra are innervated by the medial branch of the dorsalramus that issues from below that vertebra.

The global muscle system encompasses the large, superficial muscles ofthe trunk that cross multiple motion segments, and do not have directattachment to the vertebrae. These muscles are the torque generators forspinal motion, and control spinal orientation, balance the externalloads applied to the trunk, and transfer load from the thorax to thepelvis. Global muscles include the oblique internus abdominis, theobliquus externus abdmonimus, the rectus abdominus, the lateral fibersof the quadratus lumborum, and portions of the erector spinae.

Normally, load transmission is painless. Over time, dysfunction of thespinal stabilization system will lead to instability, resulting inoverloading of structures when the spine moves beyond its neutral zone.The neutral zone is the range of intervertebral motion, measured from aneutral position, within which the spinal motion is produced with aminimal internal resistance. High loads can lead to inflammation, discdegeneration, facet joint degeneration, and muscle fatigue. Since theendplates and annulus have a rich nerve supply, it is believed thatabnormally high loads may be a cause of pain. Load transmission to thefacets may also change with degenerative disc disease, leading to facetarthritis and facet pain.

For patients believed to have back pain due to instability, cliniciansoffer treatments intended to reduce intervertebral motion. Commonmethods of attempting to improve muscle strength and control includecore abdominal exercises, use of a stability ball, and Pilates. Spinalfusion is the standard surgical treatment for chronic back pain.Following fusion, motion is reduced across the vertebral motion segment.Dynamic stabilization implants are intended to reduce abnormal motionand load transmission of a spinal motion segment, without fusion.Categories of dynamic stabilizers include interspinous process devices,interspinous ligament devices, and pedicle screw based structures. Totaldisc replacement and artificial nucleus prostheses also aim to improvespine stability and load transmission while preserving motion.

There are a number of problems associated with current implants that aimto restore spine stabilization. First, it is difficult to achieveuniform load sharing during the entire range of motion if the locationof the optimum instant axis of rotation is not close to that of themotion segment during the entire range of motion. Second, cyclic loadingof dynamic stabilization implants may cause fatigue failure of theimplant, or the implant-bone junction (e.g. screw loosening). Third,implantation of these systems requires surgery, which may cause new painfrom adhesions, or neuroma formation. Moreover, surgery typicallyinvolves cutting or stripping ligaments, capsules, muscles, and nerveloops which will interfere with the spinal stabilization system.

SUMMARY OF THE INVENTION

It is the conjecture and surmise of the inventor that one source of painresults from mechanical instability of the back. Reaction to pain may infact induce further instabilities within the back, setting up an overalldecline in back health. It is the conjecture and surmise of the inventorthat episodic electrical stimulation of particular groups of muscles andassociated nerves, ligaments, or joint capsules within the lower backcan both reduce the severity of pain and reduce the frequency of painexacerbations by enhancing stability of the mechanical structures of thelower back. The stimulation may serve to “train” the muscles and improvethe tone, endurance, and strength of the muscles. The stimulation mayalso alter the stiffness of the back acutely during stimulation. Thestimulation may also improve voluntary or involuntary motor control ofmuscles involved in spinal stabilization. The stimulation may alsoimprove reflex arc activity between mechanoreceptors embedded withinmuscles, ligaments, or joint capsules, and the spinal cord, therebyenabling quick stabilization of the spinal column in the event ofunexpected loads or movements. The stimulation may also be used toinhibit muscle spasticity and muscle spasm. Protocols have beendeveloped for the use of muscle stimulation to accomplish each of theseobjectives to treat a variety of clinical disorders.

The device will be at least partially implanted and include a leadsystem that may be coupled to a boney structure within the back toprovide stable placement of the lead. The lead or leads will likely beplaced in muscle and the electrical stimulation will activate the nervesassociated with the muscle. In another embodiment, the lead willdirectly stimulate target muscles which attach directly to the spinalcolumn, without causing intentional activation of efferent nerve fibers.Target muscles for lead placement include muscles associated with localsegmental control of a motion segment, including the deep multifidus,and transversus (or transverse) abdominis. Target muscles may alsoinclude muscles such as the diaphragm or pelvic floor muscles, that whenstimulated, cause contraction of local segmental muscles.

The optimal location is not known. It is expected that some musclefibers will be activated and that some nerve fibers will be activated.An induced contraction is expected to activate stretch receptors in theback. This may cause further muscle contractions via intact neuralpathways. The result of a neuromuscular stimulus is expected to triggera cascade of reactions.

In another embodiment, leads may be placed adjacent to receptors which,when stimulated, result in reflex contraction or increased tone of thelocal segmental control muscles. Target sites for stimulation includethe disc annulus, facet capsule, interspinous ligament, supraspinousligament, and sacro-iliac joint. Electrical stimulation may be used toactivate monosynaptic stretch reflexes, which involve stretch of amuscle spindle generating an afferent impulse from the receptor regionof the spindles to excite the alpha motoneurons in the same muscle,resulting in contraction. Electrical stimulation may be used to activateshort-latency reflexes in the paraspinal muscles, which activate theparaspinal muscles en-masse. Electrical stimulation may be used tostimulate afferent fibers of distant musculoskeletal structures, such asthe limbs, which may result in contraction of local segmental muscles inthe spine. Electrical stimulation may be used to activate long-loopreflexes, which involve information processing at higher levels of thenervous system, including transcortical mechanisms.

Stimulation parameters for restoring muscle endurance, strength, ormotor control are known in the field of physical medicine andrehabilitation, where electrical stimulation is used as a therapeutictool to preserve or restore muscle function during immobilization, or indiseased, deinnervated, or atrophied muscle. It is anticipated thatembodiments of our device may use stimulation parameters (e.g. pulseamplitude, frequency, width, duration, duty cycle, shape, ramp times,wave forms, voltage) that have been previously used in these otherapplications.

Electrical stimulation has been used to help patients improve their ownvoluntary control of apparently paralyzed muscles. For example, musclestimulation has been used to improve motor control of limbs andswallowing function in patients following stroke or head injury.Facilitation is a term used to describe a treatment program aimed atenhancing volitional motor control for a patient who is experiencingdysfunction in the central nervous system. In patients recovering fromcentral nervous system insult or partial denervation of muscle, theability of the patient to “find” and contract the muscle is compromised.Neuromuscular facilitation implies the use of stimulation to augmentvoluntary efforts. One embodiment of a device for facilitation of spinalstabilization would use a sensor of multifidus activity (or activity ofanother muscle of the local muscle system) that would trigger apiggyback stimulation on top of a patient's native contraction. Stimulusamplitudes may be set to achieve a strong muscle contraction, or may beused simply as a sensory reminder. As the patient's motor controlimproves, stimulation amplitude may be decreased to provide only asensory cue as to when the multifidus should contract.

Reeducation is a term used to describe a treatment program to enhancemotor control for those patients who have altered voluntary recruitmentof the muscular system due to some peripheral change, such as pain ordisuse. Motor facilitation and reeducation are basically the samestimulation program, applied to a different patient population. Surgery,pain, trauma, disc herniation, or edema may make it difficult for apatient with an intact central nervous system to recruit a localstabilization muscle, such as the multifidus. This difficulty may beattributed to active inhibition of the motoneurons of the affectedmuscles through pain afferents. Inhibition, from whatever source, maylower the excitability of the motoneurons sufficiently to make themdifficult to recruit. Electrical stimulation activates the large nervefibers, preferentially. Thus stimulation near the nerve supply to themultifidus will recruit the large, Ia spindle afferents before visiblemuscle contraction is achieved. The effect of this Ia activation is tofacilitate the motoneurons of the same motor pool, increasingexcitability of the motor neuron pool through partial depolarization ofthe hyperpolarized, inhibited motoneurons. In addition, electricalstimulation can be used to supply sensory input about how a desiredmultifidus muscle contraction should feel, so that eventually a patientcan do so voluntarily. It is anticipated that embodiments of our devicemay use stimulation parameters to improve motor control of spinestabilization muscles.

It is believed that some patients with back pain have inappropriatelyactive global muscles, and that restoration of normal spinestabilization will require that these global muscles remain quiet,unless needed for the generation of torque. One embodiment of theinvention involves use of stimulation parameters to inhibit contractionof global muscles through direct stimulation of the target muscle, orindirect stimulation via its motor nerve fiber. Another embodiment ofthe invention would stimulate antagonist muscles for the inhibition ofagonist muscle contraction. This reciprocal inhibition is due tostimulation of the Ia afferents, which subsequently inhibit antagonistmotoneurons. Another embodiment of the invention to inhibit abnormalcontraction may rely on the phenomenon known as sensory habituation. Lowamplitude stimulation given repeatedly has been reported to reducemuscle spasticity, given at either low frequency or high frequency. Itis believed that the central nervous system allows repetitiveinformation to be suppressed, and in the process, other related neuralsystems also come under inhibition. Stimulation is done at sensorylevels only, for several hours at a time, on either the side of themuscle spasm, or the opposite side.

EMG sensing may be included in the implanted device to monitor and tomodulate the degree of muscle activity of specific muscles of the back,and the training protocol for specific muscle groups. Other sensors maybe used, such as pressure sensors, motion sensors, and nerve conductionsensors. The therapy can be titrated to obtain ideal support or controlof the back to alleviate pain. The net result of the therapy is therestoration of muscle and motor control function to improve stability,and reduce the severity and recurrence of otherwise chronic or recurringback pain, or leg pain in the case of spinal stensosis. The device canbe used as a standalone therapy or adjunctive to other procedures. Inthis context the inventor notes that disectomy and fusion operations,and procedures to insert medical device implants weaken, displace, andinjure the muscles, ligaments, and nerves near the surgical site.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the muscle and nerve stimulation targetswithin the back of a patient. The figure is divided into three panelsFIG. 1A, FIG. 1B and FIG. 1C.

FIG. 2 is a schematic view of an exemplary implementation of a system tocarry out the invention.

FIG. 3 is a graph depicting potential observable mechanical improvementsto back function as a result of the therapeutic stimulation regime.

FIG. 4 is a flow chart representing a step wise method of carrying outan exemplary embodiment of the invention.

FIG. 5 is a schematic view of potential stimulation sites on ligaments,which may lead to increased tone of the spine stabilization muscles.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of the back and in particularshows the connection of three sets of multifidus (MF) muscle fascicleslabeled 14, 16 and 18 in panel of FIG. 1A, FIG. 1B and FIG. 1Crespectively. In each panel the identified multifidus muscles areportrayed in a lateral view attached to a set of spinal vertebrae. Notethat MF fascicles 14 arising from the spinous process and common tendonof L1 (the first lumbar vertebra) 13 insert into the mamillary processesof L4, L5, and S1, and the posterior superior iliac spine. Likewise MFfascicles 16 arising from the base of the spinous process and commontendon of L2 insert into the mamillary processes of L5 and S1, theposterior superior iliac spine, and the iliac crest. MF fascicles 18arising from the base of the spinous process and common tendon of L3insert into the mamillary process of the sacrum, and a narrow areaextending caudally from the caudal extent of the posterior superioriliac spine to the lateral edge of the sacrum.

Although shown in isolation the MF fascicles overlap and cooperate toimpose stability and strength to the back. It is recognized that thespinal stabilization system consist of three subsystems: 1) the spinalcolumn consisting of sets of vertically stacked vertebral bodiestypified by vertebrae and the associated ligaments and intervertebraldiscs (not shown), to provide intrinsic mechanical stability; 2) spinalmuscles surrounding the spinal column to provide dynamic stability; and3) the neuromotor control unit to evaluate and determine requirementsfor stability via a coordinated muscle response. In patients with afunctional stabilization system, the three subsystems work together toprovide mechanical stability. In panel FIG. 1C the implanted pulsegenerator 10 is shown near the lead system 12 that places electrodes(not shown) within the multifidus muscle structures.

FIG. 3 is a graph displaying displacement as a function of load for aspine. The stability of a spinal motion segment may be measured andpresented as seen in FIG. 3. With a load applied to the spinal columnthe normal motion segment exhibits a range of motion (ROM) 30. Theresponse is non linear overall but at a neutral point a neutral zone(NZ) 32 of the response is approximately linear. The neutral zone is aregion of laxity around the neutral resting position of a spinal motionsegment, where little resistance is offered.

The applicant believes that the neutral zone is a parameter thatcorrelates well with instability of the spinal system. It has been foundto increase with back injury, muscle weakness, and degeneration. Thegoal of the therapy is to drive this measured load response to anarrower NZ by improving muscle function. The applicant surmises thatthe displacement of vertebrae can be detected correlating to normal andabnormal NZ using presently available motion tracking or positionsensing devices that are well known in the art.

It is the goal of the therapy to stimulate these MF muscles to trainthem. This training should lead to a more normal NZ. For example, apretreatment NZ 32 should improve to a more normal NZ 37 post treatment.

In FIG. 1C, a representative implementation of the invention may becarried out with a fully implanted pulse generator (IPG) 10 coupled toan implanted lead system 12. Together the IPG 10 and lead system 12stimulate the MF muscle, as shown in the panels of the figure.

FIG. 2 shows a percutaneous lead placement with a lead system attachedto a boney structure of a vertebra, more particularly on the spinousprocess 24 although the transverse process, the lamina or the vertebralbody are candidate anchor points as well. It is expected that thetransverse process will be the optimal location since the medial branchof the dorsal root of the spinal nerve courses over the transverseprocess. Although the primary targets of the stimulation are the deepfibers of lumbar multifidus 26, there are complicated mechanical andneural relations between this relatively large muscle group andcompanion groups. For this reason it is possible that additionalcandidate targets will include alone or in combinations other musclegroups including the quadratus lumborum, the erector spinae, psoasmajor, and transverse abdominis, or connective tissue such as theannulus or facet capsule that when stimulated will cause reflexcontraction of a spinal muscle. In FIG. 2 a fully implanted IPG basedsystem is shown with a pulse generator 10 coupled to an implanted leadsystem 12. As an alternative a hybrid system is shown. In the hybridsystem an external transmitter 20 couples to an implanted receiverstimulator 22.

Although direct implantation of stimulation electrodes in muscle tissueis anticipated, the purpose of the stimulation is to depolarizeinnervated sections of the muscle that will then propagate adepolarization stimulus along the nerve fibers recruiting muscle tissueremote from the site of stimulation. Induced motions and tensions inmuscle may activate stretch receptors and result in a cascade responseof related muscle groups. Transvenous stimulation may be possible aswell, however vessels of a suitable size for lead placement are notalways found in desirable locations near the target stimulation site.

Stimulation electrodes may be used to modulate nerve activity, includinginhibiting nerve conduction, improving nerve conduction, and improvingmuscle activity. It is expected that stimulation parameters will bedeveloped experimentally with animal models and most likely humanstudies. The literature and clinical studies suggest that the energylevels for stimulation are well within the energy levels produced bymodern pacing devices. The strength of the stimuli and duration of thestimulation are expected to improve the strength, endurance, or motorcontrol of the muscles, thereby reducing instability of the back.

FIG. 4 shows a flowchart describing a process or method carried out withthe implantable pulse generator. Modern programmable devices are wellknown and the flowchart is sufficient to enable one to carry out thisembodiment of the invention. In step 50 the patient is diagnosed with adefect in the spinal muscle or motor control system. The patient mayexhibit an inappropriate response as depicted by curve 33 in FIG. 3. Atstep 52 electrical stimulation is given to the muscle fascicles andrelated structures. The timing, magnitude and duration of the treatmentwill need to be titrated for the patient. In step 54 the patient istested and if the dysfunction is resolved as indicated perhaps by MRI,ultrasound, EMG, physical examination, tissue biopsy or improvedstability evidenced by curve 31 of FIG. 3 then the stimulation and trainis stopped. Otherwise the treatment continues.

FIG. 5 shows the location of ligaments coupling bony structures of thespine. It is expected that electrical stimulation of these elements vialead system 12 will facilitate the therapy. As can be seen in the figurethe ligaments lie close to the bone. The lead system 12 is placed alongthe bony structures and in an embodiment is anchored to the bone. Bothactive and passive fixation anchors are contemplated as well as gluebased fixation features. Each of the ligaments and joint capsulesenumerated in FIG. 5 are targets for stimulation.

DEFINITIONS

There is no well accepted metric for expressing all of the physicalchanges brought about by the therapeutic stimulation regime disclosedherein. For this reason the following terms are defined herein asfollows:

Stiffness is a measure of resistance to displacement when a force isapplied.

Strength is the force generating capacity of a muscle.

Endurance is resistance to fatigue.

Motor control is the ability of a patient to activate a muscle.

Muscle function refers to strength, endurance, or motor control.Improving one of these three variables, alone or in combination, canimprove muscle function to accomplish a specific task.

It is desirable to provide real time therapy in an ambulatory patient.This is best achieved by a fully implantable system. Alternatively, ahybrid system with an implanted component communicating with an externalstimulator may be best suited for intermittent use for example withsupine patients at times of rest.

1. A method of treating a patient's back comprising: generating acurrent within in a muscle of a patient at a location and of anamplitude sufficient to cause at least one muscle of the local spinestabilization system to contract; thereby, improving the function of thelocal spine stabilization muscle. 2.-33. (canceled)